1. Field
The embodiments described herein relate generally to radiation-based imaging systems. More particularly, the described embodiments relate to calibration of radiation-based imaging systems used in conjunction with radiation therapy.
2. Description
A linear accelerator produces electrons or photons having particular energies. In one common application, a linear accelerator generates a radiation beam and directs the beam toward a target area of a patient. The beam is intended to destroy cells within the target area by causing ionizations within the cells or other radiation-induced cell damage.
Radiation treatment plans are intended to maximize radiation delivered to a target while minimizing radiation delivered to healthy tissue. To design a radiation treatment plan, a designer must assume that relevant portions of a patient will be in particular positions relative to a linear accelerator during delivery of the treatment radiation. The goals of maximizing target radiation and minimizing healthy tissue radiation may not be achieved if the relevant portions are not positioned in accordance with the treatment plan during delivery of the radiation. More specifically, errors in positioning the patient can cause the delivery of low radiation doses to tumors and high radiation doses to sensitive healthy tissue. The potential for misdelivery increases with increased positioning errors.
Conventional imaging systems may be used to verify patient positioning prior to and during the delivery of treatment radiation. Specifically, this verification is intended to confirm that relevant portions of a patient are positioned in accordance with a treatment plan. Some systems may generate, for example, a two-dimensional projection image of a patient portal by passing a radiation beam through the patient and receiving the exiting beam at an imaging system (e.g., a flat panel imager). Other systems produce three-dimensional megavoltage cone beam computed tomography (MV CBCT) images and/or three-dimensional kilovoltage cone beam computed tomography (kV CBCT) images of a patient volume prior to and/or during radiation delivery thereto. Recently-developed systems include linear/arc tomosynthesis and stationary tomosynthesis, which provide three-dimensional images based on fewer projection images than required by CBCT, but usually at poorer resolution.
In this regard, the three-dimensional images mentioned above are reconstructed from projection images using known reconstruction algorithms. The reconstruction algorithms may differ depending on the particular system used to obtain the projection images. However, each reconstruction algorithm requires knowledge of the imaging geometry parameters which were in effect during acquisition of the projection images. Imaging geometry parameters may include, but are not limited to, position of x-ray source(s), position of flat panel detector, panel tilt, panel sag, etc.
Imaging geometry parameters are calculated for an imaging system during a calibration procedure. During a typical calibration procedure, a projection image of a known phantom is acquired by the imaging system. Features of the phantom (e.g., embedded fiducials) are recognized within the projection image using feature-recognition techniques. The imaging geometry parameters are then calculated based on the locations of the features within the projection image. The manner of calculation is dependent upon the particular source-detector trajectory of the imaging system.
The above-described feature recognition and imaging geometry parameter calculation can be time-consuming and processor-intensive. Moreover, and particularly relevant to systems including more than one imaging system, the required phantom, phantom location, and/or imaging geometry parameter calculation may differ depending on the type of imaging system being calibrated.
Systems are therefore desired for efficient determination of imaging geometry parameters. Such systems may be useful for calibrating multiple imaging systems and/or multiple types of imaging systems.